Interleaved transactions no longer work

[penberg]

Pull request #100 optimized the API to share the same connection, but this now breaks interleaved transactions:

  const tx1 = await libsql.transaction('deferred')
  await tx1.execute(`SELECT 1`)

  const tx2 = await libsql.transaction('deferred')
  await tx2.execute(`SELECT 1`)

  await tx1.commit()
  await tx2.commit()